Zero-Field Overhauser Dynamic Nuclear Polarization

Overhauser dynamic nuclear polarization (O-DNP) enables the production of hyperpolarization in liquids via the dissolution of nitroxide radicals. However, the application of magnetic fields has been inevitable. In the present study, we experimentally demonstrate that water can be hyperpolarized through O-DNP at effectively zero magnetic field. Selective saturation induced by circularly polarized RF fields, generated by two orthogonal RF coils, effectively circumvents the concurrence of positive and negative polarizations that would otherwise significantly deteriorate O-DNP efficiency. Through electron-to-nuclear spectral mapping, the electron paramagnetic resonance spectrum of the nitroxide radical was recorded at a magnetic field of 2 nT. A direct comparison revealed that O-DNP with circularly polarized RF is more than 10³ times as efficient as that with linearly polarized RF. Finally, at a near-zero field, Overhauser-enhanced ¹H nuclear magnetic resonance from water was observed without the need for auxiliary aids such as sample shuttling or field cycling.